1. Field of the Invention
The present invention relates to a method and apparatus, including a system and individual system components, for concurrent processing of multiple wafers in the fabrication of integrated circuits. More particularly, the present invention provides a front end vacuum processing apparatus for a cluster tool adapted to move and process wafers in tandem.
2. Background of the Related Art
The term "cluster tool" generally refers to a modular, multichamber, integrated processing system having a central wafer handling module and a number of peripheral process chambers. Cluster tools have become generally accepted as effective and efficient equipment for manufacturing advanced microelectronic devices. Wafers are introduced into a cluster tool where they undergo a series of process steps sequentially in various process chambers to form integrated circuits or flat panel displays. The transfer of the wafers between the process chambers is typically managed by a wafer handling module located in a central transfer region. Typically, cluster tools are of two different types: single wafer processing or batch wafer processing. Single wafer processing generally refers to a chamber configuration in which a single wafer is located in a chamber for processing. Batch wafer processing generally refers to a chamber configuration in which multiple wafers are positioned on a turntable and are processed at various positions within a chamber as the turntable rotates through 360 degrees. A cluster tool configured for batch processing allows multiple wafers, typically from four (4) to seven (7) wafers, to be simultaneously processed in a single chamber.
One disadvantage of batch processing is that batch processing frequently provides poor deposition uniformity from the center of the wafer to the edge of the wafer. Process uniformity is important to obtain uniformity of deposition on the wafer. The poor uniformity of batch processing systems is a direct result of having multiple wafers being partially processed at multiple stations within a single chamber.
Single wafer processing chambers act as an alternative approach to improve process uniformity. Single wafer processing is generally considered to provide a higher degree of control over process uniformity because a single wafer is positioned in a process chamber where it undergoes a complete process step, such as a deposition step or an etch step, without having to be moved to a different position within a chamber. Furthermore, the components of a single wafer processing chamber can be positioned concentrically relative to the single wafer to enhance uniformity. FIG. 1 shows a top schematic view of a cluster tool 10 having multiple single wafer processing chambers 11 mounted thereon. A cluster tool similar to that shown in FIG. 1 is available from Applied Materials, Inc. of Santa Clara, Calif. The tool includes a pair of loadlock chambers 14 and a transfer chamber 13 having a wafer handling module 12 for moving the wafers from location to location within the system, in particular, between the multiple single wafer processing chambers 11. This particular tool is shown to accommodate up to four (4) single wafer processing chambers 11 positioned radially about the transfer chamber.
In an effort to provide a vacuum processing system that provides both uniform wafer processing and high throughput, a tandem system has been developed that provides an integrated system and process chambers that work in cooperation to incorporate single wafer architecture with batch wafer handling techniques. FIG. 2 shows a top schematic view of a cluster tool 20 having multiple tandem wafer processing chambers 22 mounted thereon. A cluster tool similar to that shown in FIG. 2 is available from Applied Materials, Inc. of Santa Clara, Calif. Overhead robots or technicians in the clean room in which the vacuum processing system operates, deliver a pair of wafer cassettes 23 to a front end staging area 21 of the system. A front-end robot 25 located on the front end staging area 21 detects the presence of wafers in the cassettes 23 and transfers the wafers from the front end staging area 21 and into the loadlock chamber 24. The loadlock chamber 24 has a wafer cassette with side-by-side wafer seats that is vertically indexed to align the wafers with the operating plane of the system robot 25. However, as the wafers must be transferred from the atmospheric environment of the clean room into the vacuum environment of the cluster tool 20, the loadlock 24 must repeatedly go through a pump-up and pump-down cycle. The wafers are placed into the loadlock 24 at atmospheric pressure and the loadlock 24 is subsequently pumped down to the required vacuum. The wafers are then processed and returned to the loadlock 24 which is then pumped up to atmospheric pressure for removal from the system. When the wafers are in the system, the tandem robot 26 in the transfer chamber moves the wafers, two at a time, sequentially from the loadlock chamber 24, through each of the process chambers 22, and back into the loadlock chamber 24. Thus, the tandem process system operation is a single wafer process system with batch wafer handling and is, therefore, able to obtain a higher degree of control over process uniformnity with higher throughput.
One problem associated with the tandem processing system is that the front end staging area 21 reduces the potential throughput of wafers. Each time a cassette or group of processed wafers is removed from the loadlock 24 and a cassette or group of new wafers is introduced into the system, the loadlock 24 must be slowly vented to atmospheric pressure before the processed wafers may be removed, and then pumped down to vacuum after the new cassette or group of wafers is placed in the loadlock, to complete the transfer operation. This wafer transfer operation causes the system to sit idle while the transfer of wafers in the loadlocks is accomplished. Single wafer processing systems overcome this wafer transfer delay by employing two separate loadlocks (as shown in FIG. 1). U.S. Pat. No. 5,186,718, entitled "Staged-Vacuum Wafer Processing System and Method" discloses the use of a staged vacuum system having a front end buffer chamber adapted to supply wafers to a single wafer process system and is incorporated herein by reference. This system employs two, independent loadlocks and is known to provide rapid transfer of wafers into the system. In these single wafer processing systems, the system is able to process the wafers in one loadlock, which is pumped down to the required vacuum, while the transfer of wafers is being made into the other loadlock. However, the width of the tandem chambers in combination with the desire to maintain a small footprint of the system to preserve clean room space, precludes the use of two side-by-side tandem loadlocks to service a tandem process system such as that shown in FIG. 2. In addition, the current four-sided design of the current tandem systems, as shown in FIG. 2, also precludes the use of two side-by-side tandem loadlocks without eliminating one of the tandem process chambers 22.
Accordingly, there is a need for a vacuum processing system that provides both uniform wafer processing and high throughput and that overcomes the throughput limitations of the present front end staging area and loadlock designs. The present invention provides a front end processing environment that replaces the front end staging area of the prior designs.